System for gathering gas from a gas field comprising a high efficient high pressure compressor

ABSTRACT

A system includes a high pressure compressor having a gas input and a gas output. The gas input is coupleable to a gas field for receiving a gathered gas with a first gas pressure, and the gas output is coupleable to a gas export system for providing the gas with a second gas pressure, which is higher than the first gas pressure, to the gas export system. The system also includes a measuring unit coupled to the gas input. The measuring unit measures an input mass flow value indicative of an input mass flow at the gas input. The system also includes a mass flow control unit that is coupled to the measuring unit for receiving the measured input mass flow value and the gas input for controlling the input mass flow such that the measured input mass flow value corresponds to a set point input mass flow value.

The present application is a §371 nationalization of PCT Application No.PCT/EP2012/059386, filed May 21, 2012, and designating the UnitedStates, which, in turn, claims the benefit of EP 11168122.7, filed onMay 30, 2011, the entire disclosures of which are incorporated byreference herein.

FIELD

The present embodiments relate to a system for gathering gas from a gasfield and to a method for controlling a gas mass flow of a system forgathering gas from a gas field.

BACKGROUND

In the technical field of gas gathering, gas is gathered fromconventional gas field types, and unconventional gas field types exist.

In conventional gas fields, the stored gas includes an initially highpressure such that further compression by external compressors is notneeded in the beginning. After years of free-flow production, a highpressure central compressor (e.g., of a centrifugal type compressor) isinstalled for another couple of years. In the end of the gas fieldlifetime, low pressure wellhead compressors are implemented andconnected to the respective wellheads of the gas field. The highpressure central compressor and the low pressure wellhead compressor areadapted for sucking the gas out of the gas field and generating a highpressure gas flow for subsequent gas export systems, such as pipelinesor gas generators.

In unconventional gas fields, such as tight gas, shell gas or coal bedmethane gas fields, the gas pressure inside the gas field decreases in ashort time interval, such that central high pressure compressors and lowpressure wellhead compressors are used from the beginning of thegathering of the gas from the gas field.

Up to now, the low pressure wellhead compressors and the high pressurecentral compressor are configured as separate and independentsubsystems. The low pressure wellhead compressors and the high pressurewellhead compressor may form independent and separated units. The highpressure central compressor is optimized for a small range of inletpressures and inlet mass flow values. The low pressure wellheadcompressors are optimized for individual ranges of respective inletpressures and mass flows. Each low pressure wellhead compressor thusproduces an output pressure and an output mass flow of gas withoutconsidering the small range of inlet pressures and mass flow of gas atwhich the high pressure central compressor works efficiently. Avariation of the gas pressure in the gas field does not affect theefficiency of the low pressure wellhead compressors dramatically,whereas the efficiency of the subsequent high pressure centralcompressor is affected dramatically during pressure changes. Thegathering of the gas is thus inefficient if the high pressure centralcompressor is not working under an efficient working point for which thehigh pressure central compressor is configured for (e.g., the inlet gaspressures and inlet mass flow of the high pressure central compressorvary).

SUMMARY

The scope of the present invention is defined solely by the appendedclaims and is not affected to any degree by the statements within thissummary.

The present embodiments may obviate one or more of the drawbacks orlimitations in the related art. For example, an efficient gas gatheringsystem for gathering gas from a gas field is provided.

A system for gathering a gas from a gas field and a method forcontrolling a gas mass flow of a system for gathering gas from a gasfield are provided.

In one embodiment, a system for gathering a gas from a gas field isprovided. The system includes a high pressure compressor having a gasinput and a gas output. The gas input is coupleable to the gas field forreceiving the gas gathered from the gas field with a first gas pressure.The gas output is coupleable to a gas export system for providing thegas with a second pressure to the second gas export system. The secondgas pressure is higher than the first gas pressure. The system alsoincludes a measuring unit that is coupled to the gas input. Themeasuring unit measures an input mass flow value that is indicative ofan input mass flow at the gas input. The system also includes a massflow control unit. The mass flow control unit is coupled to themeasuring unit for receiving the measured input mass flow value. Themass flow control unit is coupled to the gas input for controlling theinput mass flow such that the measured input mass flow value correspondsto a set point input mass flow value.

According to another embodiment, a method for controlling a gas massflow of a system for gathering gas from a gas field is provided.According to the method, an input mass flow at a gas input of a highpressure compressor is measured. The gas input receives the gas gatheredfrom the gas field with a first gas pressure. The high pressurecompressor includes a gas output for providing gas with a secondpressure to a gas export system. The second gas pressure is higher thanthe first gas pressure. An input mass flow value that is indicative ofan input mass flow at the gas input is measured by a measuring unit thatis coupled to the gas input. The measured input mass flow value isreceived by a mass flow control unit. The input mass flow is controlledby the mass flow control unit, which is coupled to the gas input. Themass flow control unit controls the input mass flow such that the inputmass flow value at the gas input corresponds to a set point input massflow value.

By the above-described system and by the above-described method forcontrolling a gas mass flow, gas (e.g., natural gas) may be gatheredfrom a conventional gas field or from an unconventional gas field, inwhich tight gas, shell gas or coal bed methane gas is gathered.

The high pressure compressor is installed between the gas field and thegas export system. The high pressure compressor is adapted for receivingthe gas from the gas field with a first pressure (e.g., approximately 10to 14 bars) and adapted to compress the gas up to a second gas pressureof approximately 60 to 90 bars used by the gas export system.

The high pressure central compressor may, for example, be of acentrifugal type compressor and is most suitable for the gas gatheringin order to generate the high gas pressure for the subsequent gas exportsystem. The high pressure compressor includes a small efficient rangeregarding the inputted mass flow and input pressure in which the highpressure compressor works efficient. Due to changes in inlet conditions,such as in the mass flow and in the inlet gas pressure, a high impact onthe operation efficiency is caused.

The gas export system includes, for example, a pipeline system fortransporting the gas to a desired destination. The export gas system mayalso include a gas power plant for generating electricity or further gastreatment systems.

The measuring unit is coupled to the gas input of the high pressurecompressor and is adapted for measuring a variety of parameters that areindicative of the input mass flow of the gas at the gas input of thehigh pressure compressor. For example, the measuring unit measures theinput mass flow value (e.g., kg/s). Additionally, the measuring unit maymeasure the first gas pressure of the gas at the gas input and thesecond gas pressure at the gas output. The measuring unit is alsoadapted for measuring the output mass flow at the gas output of the highpressure compressor. Additionally, the measuring unit may measure thecomposition of the gas as well as the temperature of the gas at the gasinput and the gas output of the high pressure compressor.

The measured values (e.g., the measured input mass flow value) areprovided to the mass flow control unit. The mass flow control unitcompares the measured values (e.g., the input mass flow value) with agiven set point input mass flow value. If there is a difference betweenthe input mass flow value and the set point input mass flow value, themass flow control unit controls the input mass flow value such that theinput mass flow value corresponds to the set point input mass flowvalue.

The set point input mass flow value defines the mass flow at the gasinput of the high pressure compressor, by which the high pressurecompressor is working at a working point of the high pressure compressorand thus is working with the highest efficiency. The working point isdependent on the pressure relation between the first gas pressure at thegas input and the second gas pressure at the gas output, the mass flowof the gas through the high pressure compressor (e.g., the mass flow atthe gas input) and the rotational speed of the high pressure compressor.

Under given first pressures and second pressures, a set point input massflow value is thus given for running the high pressure compressor at aworking point of the high pressure compressor and hence at a bestefficiency. A deviation of the input mass flow value causes a severeeffect on the efficiency of the high pressure compressor. For example,if the measured input mass flow value differs dramatically to the setpoint input mass flow value (e.g., at given first and second gaspressures), compressor surge may occur.

The mass flow control unit is adapted for reducing the deviation of themeasured input mass flow value from the given set point input mass flowvalue. The mass flow control unit may control various subsystems forinfluencing the input mass flow of the gas at the gas input. Forexample, the mass flow control unit may control upstream installed lowpressure compressors, delivery valves or a mass flow in a return flowtubing and/or a mass flow of a bypass tubing, which will be explained inmore detail below.

By one or more of the present embodiments, even when the gathered massflow of gas from a gas field varies dramatically, a predetermined anddesired input mass flow at the gas input of the high pressure compressoris adjustable by the mass flow control unit such that the high pressurecompressor is working approximately at a working point of the highpressure compressor. Hence, the efficiency of the high pressurecompressor and hence the gathering of gas from a gas field is moreefficient.

According to a further exemplary embodiment, the system includes a firstlow pressure compressor that is connectable to a first wellheadarrangement of the gas field for receiving the gas from the firstwellhead arrangement. The first low pressure compressor is connected tothe gas input for providing the gas to the gas input with a first massflow. The system also includes a second low pressure compressor that isconnectable to a second wellhead arrangement of the gas field forreceiving the gas from the second wellhead arrangement. The second lowpressure compressor is connected to the gas input for providing the gasto the gas input with the second mass flow.

The mass flow control unit is connected to: a) the first low pressurecompressor for controlling the first low pressure compressor; and b) thesecond low pressure compressor for controlling the second low pressurecompressor such that the first mass flow and the second mass flow arecontrollable for controlling the input mass flow at the gas input.

Each wellhead arrangement is connected to a borehole through which gasof the gas field is gathered. A gas field may include a plurality ofboreholes. A respective wellhead arrangement is attached to eachborehole for gathering gas. Each wellhead arrangement is connected to arespective low pressure compressor. For example, if a wellheadarrangement delivers gas with approximately 1 to 4 bars to the lowpressure compressor, the low pressure compressor increases the pressureup to approximately 5 to 20 bars. By increasing the pressure, a suckingeffect is achieved, such that the gas is sucked out of the respectiveborehole, so that the respective mass flow through the low pressurecompressor is also increased.

For the low pressure wellhead compressors, screw-type or reciprocatingcompressors may be used. These kinds of compressors are relativelyinsensitive to changes in inlet conditions. The compressors have a largeoperating range and may easily handle a wide range of inlet pressurevariations and inlet mass flows variations under which an effectiveoperation may still be provided.

For example, the measuring unit is adapted for measuring each mass flowand pressure at a respective gas input of a respective low pressurecompressor and at a gas output of a respective low pressure compressor.This measured data is sent to the mass flow control unit. The mass flowcontrol unit also receives the information of the input mass flow at thegas input of the high pressure compressor. If more or less mass flow isneeded at the gas input of the high pressure compressor, the mass flowcontrol unit controls the respective low pressure compressors (e.g., thefirst low pressure compressor and the second low pressure compressor).

For example, the mass flow control unit receives the information of theworking points of the respective low pressure compressors. For adjustingthe desired input mass flow of the high pressure compressor, each lowpressure compressor is thus controlled under consideration of respectiveworking points such that each low pressure compressors is operated moreefficiently. The overall efficiency of the system is thus increased.

Additionally or alternatively, according to a further exemplaryembodiment, the system includes a first delivery valve that isconnectable to a first wellhead arrangement of the gas field forreceiving the gas from the first wellhead arrangement. The firstdelivery valve is connected to the gas input for providing the gas tothe gas input with a further first mass flow. The system furtherincludes a second delivery valve that is connectable to a secondwellhead arrangement of the gas field for receiving the gas from thesecond wellhead arrangement. The second delivery valve is connected tothe gas input for providing the gas to the gas input with a furthersecond mass flow.

The mass flow control unit is connected to: a) the first delivery valvefor controlling the first delivery valve; and b) the second deliveryvalve for controlling the second delivery valve, such that the furtherfirst mass flow and the further second mass flow are controllable forcontrolling the input mass flow at the gas input.

The first and second delivery valves may be installed between therespective first and second wellhead arrangement and the respectivefirst and second low pressure compressors. Alternatively, the first andsecond delivery valves may be installed behind (e.g., downstream) of therespectively first and second low pressure compressors.

By controlling the respective delivery valves, the further first massflow and the further second mass flow of the gas gathered from arespective borehole is adjustable and controllable individually, suchthat a desired input mass flow of the gas at the gas input of the highpressure compressor is adjustable.

In gas gathering systems (e.g., in conventional gas gathering systems),gas may be gathered without using low pressure compressors. In thiscase, only the delivery valves are installed downstream of the wellheadarrangement in order to control mass flow from the respective wellheadarrangement to the gas input of the high pressure compressor.

According to a further exemplary embodiment, the system includes areturn flow tubing that is connected to the gas output for bleeding offa part of the gas from the gas output and is connected to the gas inputfor injecting the part of the gas in the gas input. The mass flowcontrol unit is coupled to the return flow tubing for controlling thebleeding off of the part of the gas from the gas output and forcontrolling the injecting of the part of the gas in the gas input suchthat the input mass flow at the gas input is controllable.

For example, if the measured input mass flow value is below the setpoint input mass flow value, a part of the gas may be bled off from thegas output and injected at the gas input, so that the input mass flow isincreased at the gas input. In the return flow tubing, a control valvethat is controllable by the mass flow control unit may be installed.

By using the return flow tubing, an anti-surge system is established.The mass flow control unit may act as a supervisory system and controlsthe anti-surge system. By controlling the input gas parameters (e.g.,the input mass flow of the gas) and by influencing the upstream locatedsystems, such as the low pressure compressors (e.g., wellheadcompressors), the anti-surge system for the high pressure compressor issafer and more efficient.

According to a further exemplary embodiment, the system further includesa bypass tubing that is connected to the gas input for bleeding off afurther part of the gas from the gas input. The mass flow control unitis coupled to the bypass tubing for controlling the bleeding off of thefurther part of the gas from the gas input such that the input mass flowat the gas input is controllable.

For example, if the input mass flow is higher than the set point inputmass flow value, the bypass tubing may bleed off the gas from the gasinput (e.g., to the environment or to a further use), for example, intoa gas generator. By bleeding off the gas from the gas input, the inputmass flow is thus reduced until the desired set point input mass flowvalue is adjusted.

By one or more of the present embodiments, the high pressure compressorof a gas gathering system is controlled and operated more efficiently.For example, predetermined set point input values (e.g., set point inputmass flow values) of the gas at the gas input of the high pressurecompressor are determined, so that already at the design phase, the highpressure compressor may be optimized for predetermined operating rangesin order to run more efficiently at the given set point input mass flowvalues. For example, the high pressure compressor may be configured witha certain number of stator rings, cooling systems, etc., so that alreadyat the design phase of the high pressure compressor, an optimized highpressure compressor is constructable.

By one or more of the present embodiments, optimized inlet conditionsfor the high pressure compressor are generated, for example, by adaptingand controlling the low pressure compressors. By controlling the lowpressure compressors dependent on the desired input values of the highpressure compressor, an interconnected efficient and effective gatheringsystem is achieved.

Embodiments have been described with reference to different subjectmatters. For example, some embodiments have been described withreference to an apparatus, whereas other embodiments have been describedwith reference to a method. However, a person skilled in the art willgather from the above and the following description that, unlessotherwise noted, in addition to any combination of features belonging toone type of subject matter, any combination of features relating todifferent subject matters (e.g., between features of the apparatus andfeatures of the method) is considered as to be disclosed with thisapplication.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a gas gathering system according to an exemplaryembodiment.

DETAILED DESCRIPTION

In FIG. 1, similar or identical elements are provided with the samereference signs.

FIG. 1 shows a system for gathering a gas from a gas field 170. Thesystem includes a high pressure compressor 100 that includes a gas input101 and a gas output 102.

The gas input 101 is coupleable to the gas field 170 for receiving thegas gathered from the gas field 170 with a first gas pressure p1. Thegas output 102 is coupleable to a gas export system 130 for providingthe gas with a second gas pressure p2 to the gas export system 130. Thesecond gas pressure p2 is higher than the first gas pressure p1.

A measuring unit 110 is coupled to the gas input 101. The measuring unit110 measures (e.g., by sensor elements 111) an input mass flow value(e.g., in kg/s) that is indicative of an input mass flow {dot over (m)}₁at the gas input 101.

The system also includes a mass flow control unit 120. The mass flowcontrol unit 120 is coupled to the measuring unit 110 for receiving themeasured input mass flow value and the gas input 101 for controlling theinput mass flow {dot over (m)}₁ such that the measured input mass flowvalue corresponds to a set point input mass flow value.

The measuring unit 110 may also measure further mass flow affectingparameters (e.g., the first pressure p1 and the temperature at the gasinput 101 and/or the second pressure p2, the output mass flow {dot over(m)}₂ and/or the temperature at the gas output 102).

All measured data is provided to the mass flow control unit 120. Themass flow control unit 120 may calculate based on the inputted datavalues, such as the input mass flow {dot over (m)}₁, the first pressurep1, the second pressure p2, the output mass flow {dot over (m)}₂, therespective gas temperature at the gas input 101 and the gas output 102,a set point input mass flow value (e.g., kg/s) at which the highpressure compressor 100 is operated approximately at a working point.The mass flow control unit 120 may be connected to the high pressurecompressor 100 for controlling, for example, the speed of rotation ofthe high pressure compressor 100.

In FIG. 1, the tubing that transports the gas between the system devicesare shown by the solid lines, whereas the data lines for transmittingmeasuring data and control data are shown with the dotted lines.

Besides the high pressure compressor 100, in FIG. 1, further componentsof the system for gathering gas are shown. From the gas field 170, gasmay be gathered through a plurality of boreholes. At each borehole, arespective wellhead arrangement 151-154 is attached. Downstream of eachwellhead arrangement 151-154, a respective delivery valve 161-164 iscoupled. By controlling the delivery valves 161-164, a respective massflow from a respective wellhead arrangement 151-154 to the gas input 101may thus be controllable.

In order to increase the amount of gathered gas from the gas field, inall or in some tubes between the gas input 101 and a respective firstwellhead arrangement 151-154, a respective low pressure compressor141-144 is installed. Each low pressure compressor 141-144 may increasethe pressure from approximately 1 to 4 bars to approximately 5 to 20bars.

By controlling the low pressure compressor 141-144, the mass flow of gasthat is delivered by a respective wellhead arrangement 151 to the gasinput 101 is adjustable. The mass flow control unit 120 is adapted forcontrolling the respective low pressure compressors 141-144individually, so that each low pressure compressor 141-144 may deliver apredetermined input mass flow {dot over (m)}₁ of gas to the gas input101. The mass flow control unit 120 is adapted, for example, forcontrolling the rotational speed of each low pressure compressor141-144.

In the exemplary embodiment shown in FIG. 1, four low pressurecompressors 141-144, four wellhead arrangements 151-154 and fourdelivery valves 161-164 are shown. In other exemplary embodiments, moreor fewer low pressure compressors 141-144, wellhead arrangements 151-154and delivery valves 161-164 may be installed.

A gathering group may be formed by one low pressure compressor 141-144,one wellhead arrangement 151-154 and one delivery valve 161-164. Eachgathering group is coupled to a respective borehole. All gatheringgroups deliver gas to a common collecting manifold to which the highpressure compressor 100 is coupled. A gas field may include a pluralityof boreholes. A respective gathering group is coupled to each borehole.

The system may also include a further high pressure compressor 100.First gathering groups are connected to the one high pressure compressor100, and second gathering groups are connected to the further highpressure compressor.

The high pressure compressor 100 is adapted for increasing the receivedgas at the gas input 101 from approximately 10 to 14 bars toapproximately 60 to 90 bars at the gas outlet 102. For the gas exportsystem 130, a high gas pressure that is needed, for example, for thefurther processing of the gas is provided.

As shown in FIG. 1, the system includes a return flow tubing 103 towhich a bleeder valve 104 is connected. The bleeder valve 104 iscontrolled by the mass flow control unit 120. If the measured input massflow {dot over (m)}₁ is lower than the set point input mass flow, themass flow control unit 120 controls the bleeder valve 104 such that apart of the gas is bled off from the gas outlet 102 and injected intothe gas input 101, such that the input mass flow value is increaseduntil the input mass corresponds to the set point input mass flow value.

The term “including” does not exclude other elements or steps, and “a”or “an” does not exclude a plurality. Also, elements described inassociation with different embodiments may be combined.

It is to be understood that the elements and features recited in theappended claims may be combined in different ways to produce new claimsthat likewise fall within the scope of the present invention. Thus,whereas the dependent claims appended below depend from only a singleindependent or dependent claim, it is to be understood that thesedependent claims can, alternatively, be made to depend in thealternative from any preceding or following claim, whether independentor dependent, and that such new combinations are to be understood asforming a part of the present specification.

While the present invention has been described above by reference tovarious embodiments, it should be understood that many changes andmodifications can be made to the described embodiments. It is thereforeintended that the foregoing description be regarded as illustrativerather than limiting, and that it be understood that all equivalentsand/or combinations of embodiments are intended to be included in thisdescription.

1. A system for gathering a gas from a gas field, the system comprising:a high pressure compressor comprising a gas input and a gas output,wherein the gas input is coupleable to the gas field for receiving thegas gathered from the gas field with a first gas pressure, wherein thegas output is coupleable to a gas export system for providing the gaswith a second gas pressure to the gas export system, and wherein thesecond gas pressure is higher than the first gas pressure; a measuringunit coupled to the gas input, wherein the measuring unit is configuredto measure an input mass flow value that is indicative of an input massflow at the gas input; a first low pressure compressor that isconnectable to a first wellhead arrangement of the gas field forreceiving the gas from the first wellhead arrangement, wherein the firstlow pressure compressor is connected to the gas input for providing thegas to the gas input with a first mass flow; a second low pressurecompressor that is connectable to a second wellhead arrangement of thegas field for receiving the gas from the second wellhead arrangement,wherein the second low pressure compressor is connected to the gas inputfor providing the gas to the gas input with a second mass flow; and amass flow control unit, wherein the mass flow control unit is coupled tothe measuring unit for receiving the measured input mass flow value,wherein the mass flow control unit is further connected to the first lowpressure compressor for controlling the first low pressure compressor,and the second low pressure compressor for controlling the second lowpressure compressor such that the first mass flow and the second massflow are controllable for controlling the input mass flow at the gasinput such that the measured input mass flow value corresponds to a setpoint input mass flow value.
 2. The system according to claim 1, furthercomprising: a first delivery valve that is connectable to the firstwellhead arrangement of the gas field for receiving the gas from thefirst wellhead arrangement, wherein the first delivery valve isconnected to the gas input for providing the gas to the gas input with afurther first mass flow; and a second delivery valve that is connectableto the second wellhead arrangement of the gas field for receiving thegas from the second wellhead arrangement, wherein the second deliveryvalve is connected to the gas input for providing the gas to the gasinput with a further second mass flow, wherein the mass flow controlunit is connected to the first delivery valve for controlling the firstdelivery valve and the second delivery valve for controlling the furthersecond mass flow such that the further first mass flow and the furthersecond mass flow are controllable for controlling the input mass flow atthe gas input.
 3. The system according to claim 1, further comprising: areturn flow tubing that is connected to the gas output for bleeding offa part of the gas from the gas output and is connected to the gas inputfor injecting the part of the gas in the gas input, wherein the massflow control unit is coupled to the return flow tubing for controllingthe bleeding off of the part of the gas from the gas output and forcontrolling the injecting of the part of the gas in the gas input suchthat the input mass flow at the gas input is controllable.
 4. The systemaccording to claim 1, further comprising: a bypass tubing that isconnected to the gas input for bleeding off a further part of the gasfrom the gas input, wherein the mass flow control unit is coupled to thebypass tubing for controlling the bleeding off of the further part ofthe gas from the gas input such that the input mass flow at the gasinput is controllable.
 5. A method for controlling a gas mass flow of asystem for gathering gas from a gas field, the method comprising:measuring an input mass flow at a gas input of a high pressurecompressor, wherein the gas input receives the gas gathered from the gasfield with a first gas pressure, wherein the high pressure compressorcomprises a gas output for providing gas with a second gas pressure to agas export system, and wherein the second gas pressure is higher thanthe first gas pressure; measuring, by a measuring unit that is coupledto the gas input, an input mass flow value that is indicative of aninput mass flow at the gas input; receiving, by a mass flow controlunit, the measured input mass flow value, wherein a first low pressurecompressor is connectable to a first wellhead arrangement of the gasfield for receiving the gas from the first wellhead arrangement, whereinthe first low pressure compressor is connected to the gas input forproviding the gas to the gas input with a first mass flow, wherein asecond low pressure compressor is connectable to a second wellheadarrangement of the gas field for receiving the gas from the secondwellhead arrangement, and wherein the second low pressure compressor isconnected to the gas input for providing the gas to the gas input with asecond mass flow; and controlling, by the mass flow control unit coupledto the gas input, the input mass flow, wherein the mass flow controlunit is coupled to the measuring unit for receiving the measured inputmass flow value, and wherein the mass flow control unit is furtherconnected to the first low pressure compressor for controlling the firstlow pressure compressor and the second low pressure compressor forcontrolling the second low pressure compressor such that the first massflow and the second mass flow are controllable for controlling the inputmass flow at the gas input such that the measured input mass flow valuecorresponds to a set point input mass flow value.
 6. The systemaccording to claim 1, further comprising: a return flow tubing that isconnected to the gas output for bleeding off a part of the gas from thegas output and is connected to the gas input for injecting the part ofthe gas in the gas input, wherein the mass flow control unit is coupledto the return flow tubing for controlling the bleeding off of the partof the gas from the gas output and for controlling the injecting of thepart of the gas in the gas input such that the input mass flow at thegas input is controllable.
 7. The system according to claim 2, furthercomprising: a bypass tubing that is connected to the gas input forbleeding off a further part of the gas from the gas input, wherein themass flow control unit is coupled to the bypass tubing for controllingthe bleeding off of the further part of the gas from the gas input suchthat the input mass flow at the gas input is controllable.
 8. The systemaccording to claim 3, further comprising: a bypass tubing that isconnected to the gas input for bleeding off a further part of the gasfrom the gas input, wherein the mass flow control unit is coupled to thebypass tubing for controlling the bleeding off of the further part ofthe gas from the gas input such that the input mass flow at the gasinput is controllable.